Process FOR oxidation of concentrated black liquor

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...

Utility Patent

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C162S031000, C162S047000

Utility Patent

active

06168685

ABSTRACT:

FIELD OF INVENTION
The present invention relates to the processing of spent digestion liquor from the kraft pulping process.
BACKGROUND TO THE INVENTION
In the pulping of wood by the kraft process, wood chips are digested in a pulping liquor which is an aqueous solution of sodium hydroxide and sodium sulfide (known as white liquor) to dissolve out lignin from the wood chips and free the cellulosic fibres so as to form a pulp. The wood pulp then is washed free from pulping chemicals and usually is forwarded to a bleach plant for brightening.
The spent pulping liquor from the pulping operation, known as black liquor, usually is concentrated and then is combusted in a recovery boiler to burn off its carbon content. Usually, the concentrated black liquor is heated with steam to lower its viscosity prior to combustion. The residual mass, known as smelt, then is further processed to form white liquor for recycle to the digestion step.
Since the spent pulping liquor contains sodium sulfide and other sulfur compounds which are themselves malodorous or which form hydrogen sulfide and/or other malodorous sulfur compounds, it is common practice to subject the sulfur-carbon black liquor to oxidation at some selected stage in the sequence, in order to convert the sodium sulfide and/or other sulfur compounds to more stable compounds, generally sulfates and thiosulfates.
This oxidation procedure, commonly known as BLOX (black liquor oxidation), also involves the evolution of heat and U.S. Pat. No. 4,239,589 proposes a procedure to effect recovery of this heat of reaction. In particular, this prior art locates the BLOX step between the first effect evaporation and the flash tank of the multiple effect concentration used to initially concentrate the dilute black liquor from the pulping operation to about 50% solids concentration.
This prior art also describes conventional BLOX polishing in which the concentrated black liquor is further oxidized to convert any residual oxidizable sulfur compounds to stable form and further concentration of the black liquor to about 65% solids concentration, before feeding to the recovery boiler.
The capacity of a pulp mill to produce wood pulp often is determined by the capacity of its recovery boiler. By employing the procedure of the present invention, as described below, the recovery boiler is able to accommodate larger amounts of solids throughput, thereby increasing the production capacity of the mill.
SUMMARY OF INVENTION
In accordance with the present invention, heating of the concentrated black liquor with steam prior to combustion in the recovery boiler is eliminated but rather the concentrated black liquor is preheated by exothermic oxidation of solids present in the concentrated black liquor. Oxidation of solids in this way both generates energy to heat the concentrated black liquor and decreases the heating value of the solids in the concentrated black liquor, thereby permitting a production capacity increase in the recovery furnace, theoretically equivalent to the percentage of the heating value reduction.
Accordingly, the present invention provides an improvement in a process wherein cellulosic fibrous material is digested with a pulping liquor comprising sodium sulfide and sodium hydroxide to form a pulp and a black liquor, separating the pulp from the black liquor, the black liquor first is concentrated and oxidizable sulfur compounds in the black liquor are oxidized to a stable form and then the concentrated black liquor is combusted to burn off carbonaceous material and form a smelt, and the smelt is processed to form white liquor.
The improvement provided by the present invention comprises preheating concentrated black liquor substantially free from oxidizable sulfur compounds to form preheated concentrated black liquor by exothermic oxidation of black liquor solids contained in the concentrated black liquor, whereby the preheated concentrated black liquor is provided with a decreased heating value, and feeding the preheated concentrated black liquor to the combustion step.
The oxidation process which is effected herein is carried out on concentrated black liquor which has already been oxidized by the BLOX process to convert sulfur compounds to sulfate or thiosulfate, for example, by any of the procedures described in the aforementioned U.S. Pat. No. 4,239,589. The BLOX process does not consume black liquor solids.
The oxidation process effected herein consumes carbonaceous material present in the concentrated black liquor although any residual unoxidized sulfur compounds which may be present also will be oxidized thereby.


REFERENCES:
patent: 4239589 (1980-12-01), Elton et al.
patent: 4313788 (1982-02-01), Gulley
patent: 4718978 (1988-01-01), Spannuth et al.
patent: 5061377 (1991-10-01), Lee et al .
Smook, Gary A. “Handbook for pulp & paper Tech”, 2nd edition, Angus Wilde Publications, p. 133, 1992.
Ronald McDonald, “The Pulping of Wood,” vol. I, 2ndEd., pp. 526-529, 1972.
T.M. Grace, “Increasing Recovery Boiler Throughput”, TAPPI Journal, Nov. 1984, pp. 52-58.
Mc Donald, Ronald, “The Pulping of Wood”, 2ndEdition, vol. I, 1969, McGraw Hill Book Co., pp. 614-616.
Zecchini et al., “A new black liquor oxidation system that is energy efficient”, TAPPI Journal, Jan. 1986, pp. 70-73.

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